Short-wave antenna



w 1.. m A a mo H 1 e TF N 0 NIL R 5, cf W 0 2 WM w m ww 5 DY N B A A ALFORD SHORT WAVE ANTENNA H dimensions in farms ofA March 21, 1950 Flled June 22 1946 ix w 2 m 4 a a L e 0 TL .N 0 0 NA R 5 8 E 0 w I WW w e I e E A h II S mm mfl 3 u A Dm mm o O o Ol l I I I17 I I I I I I FW 9 L 2 A W T m S Fig. 4.

Pattern March 21, 1950 Flled June 22 1946 Patented Mar. 21 1950 SHORT WAVE ANTENNA Andrewi Alliol-dpCambridge; Mass, assignor to- TheRaulandyCorporation, Chicago, 111., a corporation of Illinois.

Application June22;'1946, Serial No. 678,581

4 Claims; (Cl. 250-33) This invention relates to new and useiul 1 im it provements in antennas'for operatingon a -broad band". of horizontally-* polarized high frequency" m theart of television receiving, a number of antenna requirements" remain 1 un t satisfied. Generally speaking, it is desirable that such antennas should not have pronounced 'horizontal directivity.-= One 1: reason for is: that broadcast stations a-re not necessarily grouped" together and that," therefore; signals will not reach receivers from just one or two" d-irectiorisi Evenwhere'it might seem apparent that signals should becoming from a few directions; field strength 7 tests" might prove" otherwise,- i because some sign'als arriveindirectlyafter reflection. It is not at all 3 practical to expect that the =purchaser of a ho-me receiver will pay' for an actual survey so that he can" use a directive antenna;

Moreover, the 'average radio *shop technician is 1 not prepared to" maintain such arr-' antenna in its original optimum operating conditionor to modif'y it to pick up signals" from new transmit ting stations whichmay come 1 into being within Horizontally polarized hal'i wave" dipoles Lhave been-'commonlyusediorFMand television home Such a dipole has maxim-time hori zontald-i'rectivity in opposite directions I along a- 1ine"which is at ri'g ht angles to it and passes through its center point. There are nulls-in its receivers? horizontal radiation patternsin both #directions along its'main axis; Even strongsig'nals "are not i picked up by 'a dipolewhoseaxi's pointsat the station sending them.

Dipoles are objectionable-'as mechanicaIstruc tur es; since they "are large, it unwieldy, unsightly; and fragile; Them 'irequencyrband extends from 88 mm to *108 *mc; Aha-l-f wave dipole (le and preferably should not be used where bending stresses occur; such as the stresses *at the center of the dipole An:ordinary" dipole is a timed "element of rela tively'high Q" and," therefore; can operate efli ciently only over'ali'mited frequency band? Both- FM and television" broadcasts", however; occupy broadbands in theirequencyspectmm:

Though a horizontal dipole does have desired upward and downwardnulls sin itst-ivertical patterns; its main 'lobes, in those patterns since they; are substantially circulaituare too broad. As a" result the gain offered by a dipole to signals com-- m from the horizonsis not -v'eryw much greater. than the gain offered by it to manyasignals' coming from below thetghorizon. Theresarelcompon ents of both automobile ignition noise'andhousehold appliance noise which fallnwithin theirsquency bands of and televisions Interference of this kind which isastrong eno-ugh'vtotmattei,

usually comes from nearby and approachesroof topantennas from directions s appreciably "below th e horizom Other "interference like it '1 may; be

generated at: distancenpoints near the: horizon:

but is attenuated markedly: long betore it reaches:- the antenna; Thus interference which might be strongenough to spoiii reception "can? beieliminated by the use of a receiving antenna which is diree+- tive toward -the horizon; Such: an antennais also beneficial. for the increased? antenna gaini: it

An FM receiver, due to theiaction: oiits offers. limiter, can discriminate: against amplitude: modul'ated waves and thuscan eliminate much: noise and static ordinarily transmittedflon thisnlcind Butpfrequency. modulated signals ten of" wave;

tering the receiver mu's't bestrong enou'ghafor the li r'niter"to function The IE gain of inex' signals are'strong. It is particularly's'for such receivers that high antenna gains is important.

pensive jreceivers isofteni quite-low and,thereiore,= their limiterswill not operateunless therinput However; an inexpensive receiver i will cease tebe inexpensive if it requires a costly antermw essential that thedesired high gain antenna.

must be of simple construction; must lee-cheap to build; and i must use a' simpleand inexpensive It *is obvious; in the -light bi ths-foregoing; that t a much improved FM'and/or television receiving antenna is needed.- It iszidesirable that the improved antenna should be horizontally polarizedti in-order tofavor FM and television waves, which are horizontally polarized, and to reject all components of random interference waves whichzmayr be vertically polarized (automobile ignition noiset actually has a preponderanceof verticallypolarized components).

Itis also desirable that that improvedantenna should match an inexpensive transmission 1inewithoutelab0rate matching de i vices; that it employ amini-mu1n oi dielectric material which isstructural-ly essential that itbe small and compact; that it be substantially FY1011!- directive in horizontal planes; that it offer appreciable antenna gain; that it have directivity toward the hoizon in vertical planes; and that it have a broad frequency pass band.

The objects of this invention are to provide an antenna which is improved so as to have the features described above.

Other objects, features and advantages will be apparent to those skilled in the art from the following description taken in connection with the drawings, in which:

Fig. 1 is an isometric representation of oneembodiment of the invention;

Fig. 2 is an isometric representation of a second embodiment of the invention; I

Fig. 3 is an isometric representation of a thir embodiment of the invention;

Fig. 4 is a horizontal radiation pattern of an antenna as in Fig. 1;

Fig. 5 is a vertical radiation pattern of an antenna as in Fig. 1;

Fig. 6 is an input impedance chart which shows changes in input impedances of an antenna according to this invention as the frequency of the energy feeding it is varied in three separate ranges in the frequency spectrum. The resistive component is plotted from left to right on a horizontal axis and the reactive components are plotted vertically;

Fig. '7 is a representation of a portion of the radio frequency spectrum showing the allocation of bands thereof to television, FM and facsimile broadcasts;

Fig. 8 is a chart showing four plots of standing wave ratios vs. frequencies with respect to standing waves on a 300-ohm lead-in line feeding, respectively, a simple loop antenna and three other antennas which were progressively modified according to principles underlying this invention; and.

Fig. 9 is a curve showing corresponding values of the H and W dimensions appropriate for antennas as in Fig. 1.

I have discovered that an open-ended and lon-' gitudinally slotted cylinder which is distorted so that its transverse cross section is a fiat ellipse, and which is of the proper size and proportions, will attain these objects and that it maybe directly and simply connected to a receiver (or transmitter) by a transmission line whose conductors are connected across one end of the slot.

Fig. 1 shows that the radiating element l is an open-ended elliptical cylinder, one broad sidewall of which has a longitudinal gap 2 extending between its ends. A receiver (or transmitter) 3 is connected to cylinder I by a parallel transmission line 44. In the drawing, elliptical cylinder l is shown to have three dimensions W, D and H. W and D are, respectively, the long and short axes of the elliptical cross section of the cylinder while H is the height of the cylinder or the length of the edges of the gap extending along its wall.

The selection of these dimensions has a controlling influence upon attaining the various objects of this invention. The absolute dimensions of the elliptical cylinder as well as its relative proportions control its input impedance, its radiation patterns, and its band width.

There is a well known simple type of horizontally polarized thin loop antenna which has a perimeter of such length as to carry one com-- plete standing wave loop of voltage having a'single maximum opposite to the feed point and a single minimum at the feed point. Because of- 4 element, it behaves in many respects like a dipole. Therefore, its horizontal radiation patterns are all shaped like a figure eight and this antenna is not omni-directional. This kind of a loop can be distorted into an oval flattened shape and it will still have the same horizontal directivity. The nulls in the figures eight can be made to disappear if the perimeter 'of'the loop is decreased but, for this kind of -loop, the input impedance then becomes prohibitive. Of course,

. auxiliary matching means can be used to adapt the antenna which has thus been decreased in size to a particular frequency but such a match remains valid only'for frequencies close to that frequency and becomes very bad but a few percent away from it. Such compensation, in other words, results in a narrow pass band. Besides this, even the slightest distortion in the shape of the small compensated loop would seriously affect the adjustment and destroy the impedance match, even for that small pass band.

I have found a method whereby it is possible to reduce dimensions W and D of an antenna as in Fig. 1, thus making its elliptical perimeter small enough for the antenna to have desirable horizontal nondirectivity. Obviously, at the same time the antenna acquires desired physical compactness.

useable range and, besides, the band width remains adequate.

According to one feature of this invention the dimension W of the flattened cylinder, i. e, its

greatest dimension, is less than-one-quarter of a wave length. According to another feature of this invention, the length H of the flattened cyl-- inder can also be of the order of one-quarter of a wave length, and usually will be less. Considering W to be the width ofthe cylinder; D to be its depth; and H its height; the ratio W/H, e. g. width-to-height, may vary from one-to-one to two-to-one, and the ratio W/D, e. g. width-to depth, may vary from two-to-one to six-to-one. Antennas proportioned-within those ranges and having a gap a small fraction of a wave length wide-.0125 7\ was tried'and found satisfactorycan be built to operate efiiciently with inexpensive transmission lines in either of two convenient input impedance regions without elaborate match- V ing devices.

Referring to Fig. 8, plot I therein shows that for an antenna, which may also be considered to be designated I, a satisfactory standing wave ratio,

inordinately large in parts of this frequency band and that, accordingly, the antenna became a narrow band device requiring compensation to attain even a small operating range. The curve of antenna I after its perimeter was reduced is not shown.

Plot II of Fig.8 is a curve, corresponding tov curve I, fora loop antenna of somewhat re duced perimeter whose H dimension was in-- creased by a predeterminedamount- The modification in its perimeter, as expected, reduced itsv horizontal directivity while the change in dimen--'. sion H preserved the input impedance for almost. its complete pass band. This is shownin the"- This reduction of the perimeter is made possible by increasing dimension H. When H is increased the input impedance stays within a tastiest By use, fast the standing wave remains satisfactory from about 88 inc. to about 148 me.

riots III and Iv show the erformance or ante nas in which the modifications described above were carried progressively further. Antenna III is, in fact, the embodiment shown in Though the operating band widths of an tennis III and IV have been materially reduced,- they are adequate to cover the combined bands allocated to FM and FM television and facsimile; on the one hand, and to the upper television band shown in Fig. 7 on the other hand.

' single antenna of identical proportions in te of x but in which x corresponds to the mid ,ueney, 19'6 of the television band, 176 me. to 216 incp, would have a band width of as proximately 1.5 (ratio or highest to lowest fie quency) without compensation whereas that tele vis on band has a width or only 1.2.

In the antennas modified as described above. i "e. the antennas related to plots II, III, and IV of Fig. 8, it was found that control over both. their fadiation patterns and their input 1mpedances could be exercised by distorting andflattoning them into an oval or elliptical shape. Therefore, these combined modifications and design principles permit construction of an an tenna having the necessary polarizations; thedesired directivity; and an appropriate input impedance which remains relatively constant throughout an adequate pass band.

Antennas proportioned and designed according to these instructions have been found to re-'- qei'v'e satisfactorily frequency modulated signals through the band from 88 me. to 108 inc; and also t'elevi'sion "signals in "the band from 176 md. to 2T6 he. Antennas for receiving on these two bands have slightly different dimensions but when proportioned with respect to the wave lengths corresponding to the midfrequencies of thse respective pass bands they operate successfully over these bands. In this connection, it is no 'd that whenjin the specification and claims, reference is made to frequency and to wave length, the center frequency of a band, "or the Wave length corresponding to it, is meant-unless dine other meaning is indicated in the context.

Aiiothr tenure of antennas constructed ac cording to the present invention is that they afford two-to-on'e impedance matches ror transmission lines readily available on the market. One embodiment can be connected directly to an 80 ohm parallel transmission line with no impedance matching devices of any kind. Another embodiment can be connected to another well known type of inexpensive parallel transmission line, the BOO-ohm line, provided that its conductors are gradually spread apart, as they approach their points of connection to the antehna, to form a quarter wave length transformer effecting a gradual transformation to increase the characteristic impedance of the line to 460 ohms. 'Such an embodiment is shown in Fig. 2 Since the antenna is like that of Fig. 1 and the feed means are conventional, "this Fig. will his be described in detail and no reference numbers are 'usedih it. Th'equarter wave length tapered transformer would be 30 inches long if the enterfreduency were 100 "mc.

L explained above the proportions of these may *be anywheres Within cftaihflhdibted rtinges. However, "it "may be helpful to know that certain antennas of this type. were? tested with especially good results and they'hnd? the following specific dimensions: An antenna with an input impedance in the high impedancethr'oiigh a quarter wave length'transformerthis antenna performs satisfactorily over a band width of 1.25. i

An antenna with an input impedance Pin "the low impedance region which was connected directly to an -ohm line had the somewhat wider band width of 1.5. Its measurements "-vneti'e W:.3'55 \;H=.238 and D=.05J\. For a midfrequency of me. these measurements in inches would be W=42"; 11:28"; and D=6". I

Cylinder I should be made at-least in part-of conducting material and its surf-acesmust-be effectivel'y continuous electrically. However, its surfaces need not necessarily be physically continuous and, for example, may be of wire netting. An embodiment constructed as shown in Fig. 3 consisted of three elliptical conductive hoops each shaped like a transverse cross section of the solid sheet metal antenna shownzin Fig. 1. The top and bottom hoops correspond. respectively, to the edges of upper and lower ends of the cylinder of Fig. 1, and'the midd'le hoop corresponds to a transverse sectionof the central portion thereof. The open endsof-these hoops are interconnected by the wires corresponding to the edges of gap 2. In the structure of Fig. 3 the hoops are numbered 5, hand I and the interconnecting wires are numbered 9 and I0.

Antennas of the types shown in Figs. 1, 2ZEY1d 3' may be attached to any suitable support, e. g. to a steel mast along the line diametrically "opposite to and parallel with thecenter-line'voi gape.

1 Fig.4 shows that the'horizontal radiati'on' 'pah tern of an antenna accordingto Fig. l is of a very desirable kind of reception in or near to a metropolitan area where several broadcasting stations are located. I The antenna is installed so that the gap extends vertically. The gapmay be faced inthe direction of the weakest desired station since it is approximatelyin that direction that the antenna is most sensitive. As already explained, however, signals from all directions will be received including those which reach the antenna on its sides which are least sensitive.

The antenna of Fig. S'has-appreciabIe dime-- tivity toward the horizonand, therefore; -hes greater gain thanotherwise. Moi'eoverfiif itwis installed about ten feet over a conductive' :ro'of this directivity will -increase an-ct upwaidmami changed, so that it has no reactive components at all at two points. At two particular frequencies in this region the inputimpedance is purely resistive being, respectively, near to 100 and 150 ohms. For an appreciable band width in this region the reactive component is never very large and the resistive component remains substan tially between 89 and 150 ohms. Therefore, in this region the, antenna may be conveniently used with a low impedance line. In fact, the antenna presents a satisfactory match, e. g. never greater than two-to-one, to an 8(l-ohm line and has a satisfactory band width Within this region. It will be noted that there is another frequency range, curve C, in which the impedance curve again crosses the abscissa of this chart. At this point the input impedance once more becomes purely resistive and is about .460 ohms. For a frequency range which is adequate to cover an existing transmission band in that region the impedance remains sufficiently constant and can easil be matched to a 300-ohm parallel transmission line in the manner described above and shown in Fig. 2.

.Fig. 7 is included in the drawing as a convenient reference to show transmission bands which have been allocated to television, FM and facsimile transmissions, inasmuch as the antenna herein is intended for FM and television.

Fig. 9 shows the relationship between dimen sions H and W of differently proportioned antennasv of the type shown in Fig. l which have substantially the same lower frequency limit as determined by their input impedances in rela- 'tion to, the output impedance of a 300-ohm feed .line. This figure has. been plotted from actual values of W and H in terms of the wave length of the lowest frequency. Antennas whose H-to- W or W-to-H ratios fall on the curve of Fig. 9 will generally have satisfactory input impedances :as explained above. Those having relatively low values of H will have horizontal patterns approximating figures-of-eight with deep nulls while those with relatively large values of H have more satisfactory, nearly circular horizontal patterns in accordance with principles explained above.

The theory of operation of my antenna has not as yet. been completely worked out. However, as shown above, several principles governing its operation can be explained and its design factors have been worked out in detail. In the following part of this application certain additional considerations also bearing generall on the problem are touched upon.

. Since preferably nothing should be received from below an FM or television antenna, and nothing needbe received from above it, substantially equal RF currents should be caused to flow in opposite directions in parallel radiating portions of it, these portions, of course, lying in one or more horizontal planes. Moreover, these elee merits must be bent into something approximating an oval or rectangular 1001) since two straight .iopposing horizontal elements would result in un- Fill desirable nulls in horizontal patterns. It is not. absolutely essential to employ an oval'or rectan gular shape if an antenna with only a down- -ward-null (without an upward null) is desired.

Such an antenna would have less gain than one with both downward and upward mills and for this reason should not be preferred. l

I have ascertained that generally the bandwidth of a loop depends in part on three factors: (a) the area included with the loop; (11) the width of the conductive structure comprising the loop; and (c) the degree of compensation, e. g.

the extent to which band stretching circuits are,

used.

Both the radiation resistances and the band widths of small loops vary approximately as the fourth power of their diameters and, therefore, small loops have small band widths and must be variably tuned to be useful over a broad frequency band.

As indicated above, by increasing the size of the conductive radiating element, the thickness of the wire or the width of the metal strip of the loop, it is possible to decrease its perimeter without causing prohibitive changes in the input impedance and, therefore, by this means, a small loop can be matched to an ordinary transmission line. By this single expedient, compactness, matching and directional properties are all obtained.

Moreover, distortions in the shape of this thick ened type of antenna, e. g. distorting it'so that its transverse cross section is oval or elliptical, further changes both its input impedance and its radiation patterns.

These increases in the size of the conductive structure also increase its vertical dimension and thus somewhat concentrate the distribution of energyin vertical planes and increase horizontal directivity in them. 3

If gap 2 should be located in flattened cylinder l in the plane containing the long axes of its elliptical cross sections the antenna would receive vertically polarized waves (or transmit them) almost as well as horizontal waves. This would adversely affect the signal to noise ratio and, therefore, and for reasons explained in detail above, this construction is undesirable. This would be true even though cylinder 1 were not very flattened and the ratio of the long and short axes of its elliptical cross sections were near to I.

What I claim is:

l. A horizontally polarized antenna which has substantial omnidirectivity in horizontal planes, comprising a flattened tubular radiator which is from 2 to 6 times as wide as it is deep and from 1 to 2 times as wide as it is high, the width of the tube as measured in terms of a wavelength corre-. sponding to the center frequency of its operating band being less than one quarter of said wavelength, a longitudinal gap extending between the open ends of the radiator in the center of one of its wide side walls parallel to the axis of the radiator, the gap having a width substantially of one hundredth of said wavelength, the exterior surface of the radiator being effectively continuously conductive for the highest frequency in said band, and two feed points respectively located opposite to each other on the edges of the gap for connection to the conductors of a between 1.25 and 1.5 parallel-conductor transmission lines of both 80 and 460 ohms surge impedance, comprising an open ended cylinder which has an elliptical transverse cross-section and is from 2 to 6 times as wide as it is deep and from 1 to 2 times as Wide as it is high, the height of the cylinder being a little less than a quarter of one Wavelength corresponding to the center frequency of the operating band, a longitudinal slot extending between the ends of the cylinder in the center of one of its wide side walls parallel to the longitudinal axis of the cylinder, the gap having a width substantially of one hundredth of one of said wavelengths, the exterior surface of the cylinder being effectively continuouslyconductive for the highest frequency in said operating band, and the respective points of connection for the conductors of the transmission line being oppositely positioned points on the edges of the gap.

3. A horizontally polarized antenna which has substantial omnidirectivity in horizontal planes and substantially matches over a band having a ratio of highest to lowest frequency width of 1.5 an 80 ohm parallel-conductor transmission line comprising an open-ended cylinder which has an elliptical transverse cross-section and is .355 wavelength wide, .05 wavelength deep and .238 wavelength high, the cylinder having a longitudinal gap extending between its ends in the center of one of its wide side walls parallel to the longitudinal axis of the cylinder, the gap having a width of .0125 wavelength, the exterior surface of the cylinder being effectively continuously-conductive for the highest frequency in its operating band, said wavelength being that of the center frequency of said operating band, and the respective points of connection for the conductors of the transmission line being oppositely positioned points on the edges of the gap.

4. A horizontally polarized antenna which has substantial omnidirectivity in horizontal planes and substantially matches over a band having a ratio of highest to lowest frequency width of 1.25 an end of a 300 ohm parallel transmission line which has its conductors spread over a distance at least as great as a quarter wavelength. corresponding to the center frequency of the operating band of the antenna to increase the surge impedance of the transmission line by substantially 50 percent, comprising an open-ended cylinder which has an elliptical transverse crosssection and is .245 wavelength long, .06 wavelength wide, and .176 wavelength high, the cylinder having a longitudinal gap extending between its ends in the center of one of its wide side walls parallel with the longitudinal axis of the cylinder, the gap having a width of .0125 of said wavelength, the exterior surface of the cylinder being effectively continuously-eonductive for frequencies in said operating band, and the respective points of connection for the spread conductors of the transmission line being oppositely positioned points on the edges of the gap.

ANDREW ALFORD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,234,293 Usselman Mar. 11, 1941 2,238,770 Blumlein Apr. 15, 1941 2,400,867 Lindenblad May 21, 1946 FOREIGN PATENTS Number Country Date 709,543 France Aug. 7, 1931 

